1. Field of the Invention
The invention relates in general to an encapsulation structure of organic electroluminescence device (OELD) and, more particularly, to the encapsulation structure of OELD being resistant to moisture and stress releasable.
2. Description of the Related Art
Use of organic electroluminescence device (OELD) in the flat panel displays possesses several competitive advantages, such as self illumination, high brightness, wide viewing angle, vivid contrast, quick response, broad range of operating temperature, high luminous efficiency and uncomplicated process of fabrication. Thus, the OELD represents a promising technology for display applications and has receives the worldwide attention in the recent years.
The typical structure of OELD is mainly constructed by interposing an organic light emitting layer between an anode and a cathode. A hole injection layer (HIL) and a hole transport layer (HTL) are interposed between the anode and the organic light emitting layer. An electron transport layer (ETL) is interposed between the cathode and the organic light emitting layer. This laminated structure of OELD facilitates the electron flow from the cathode to the anode. The organic light emitting layer can be divided into tow groups according to the materials in use. One group is a molecule-based light emitting diode, substantially comprising the dyestuffs or pigments, and so called as “OLED” (i.e. organic light emitting diode) or “OEL” (i.e. organic electroluminescence). The other group is a polymer-based light emitting diode, so called as “PLED” (i.e. polymer light emitting diode) or “LEP” (i.e. light emitting polymer). Whether the OLED or the PLED is applied in the organic electroluminescence device, the moisture shock has the considerable effect on the image quality displayed by the device. Those common effects includes the short emitting life, low emitting efficiency of the organic light emitting layer caused by the material degradation, and the “dark spot” (i.e. the spot of the light emitting portion unable to emit the light which results from a lack of current flow at the spot) caused by the adhesion failure between the organic light emitting layer and the cathode. Therefore, it is necessary to protect the organic electroluminescence device from the moisture intrusion.
FIG. 1 illustrates a conventional structure for protecting the organic electroluminescence device. A lamination structure 6, formed on a substrate 2, comprises an anode 3 (made of indium tin oxide (ITO), indium zinc oxide (IZO) or cadmium tin oxide (CTO)), an organic light emitting layer 4 and a cathode 5. Also, a transparent casing such as a glass sealing case 7 is assembled with the substrate 2, thereby providing an internal space 10 between the glass sealing case 7 and the lamination structure 6. The solid desiccating agent 8 and dried inert gas are enclosed in the internal space 10. The solid desiccating agent 8 absorbs moisture and maintains the solid state even after absorbing the moisture. The dried inert gas is used for isolating the solid desiccating agent 8 from the lamination structure 6. Although this conventional structure of FIG. 1 does ease the moisture shock to the organic electroluminescence device, it is too bulky to be practical for the future displayer design (i.e. can not meet the thin, light and portable requirements of the future product). Also, the stress generated during the fabricating process and accumulated in the conventional structure of FIG. 1 cannot be released due to the rigid components for composing the structure.
FIG. 2 illustrates the other conventional structure for protecting the organic electroluminescence device. The electroluminescence unit U includes an anode 11, a hole transport layer (HTL) 12, a light emitting layer 14, a electron transport layer 16 and a cathode 18. The electroluminescence unit U is covered with a protective structure comprising an organic barrier layer 20 and an inorganic barrier layer 22. The electroluminescence unit U and the protective structure are preferably supported by a substrate 24. Both of the organic barrier layer 20 and the inorganic barrier layer 22 are made of the materials being resistant to moisture to an extent (such as a cured resin), so as to form a double-layer structure for protecting the electroluminescence unit U. However, the organic barrier layer 20 is still moisture permeable. The moisture could invade the electroluminescence unit U through the exposed edges of the organic barrier layer 20, or through the interface between the organic barrier layer 20 and the substrate 24. Moreover, because the stress generated during the fabricating process and accumulated in the structure of FIG. 2 cannot be well released, it is impossible to form a pinhole-free and micro crack-free inorganic barrier layer 22. Even the double-layer protection (including the organic barrier layer and the defective inorganic barrier layer 22 with the pinholes and micro cracks) is applied in the structure of FIG. 2, the moisture still penetrates and cause damage to the electroluminescence unit U.
Another conventional structure, found in U.S. Pat. No. 6,268,695 by Affinito, disclosed a flexible environmental barrier used on both sides of an organic electroluminescence device. The flexible environmental barrier may be a foundation, a cover, or a combination thereof (which is preferred). Either or both of the foundation and/or the cover may have multiple layers (e.g. several organic and inorganic barrier layer laminated one by one), for ensuring the moisture-proof of the flexible environmental barrier. However, long fabrication time and large amount of raw materials are needed for making the multiple barrier layers. Therefore, fabrication of the structure provided by Affinito is time-consuming and high production cost.